This invention relates to a method for purifying molybdenum which involves precipitating magnesium-ammonium salts of arsenic and phosphorus from an ammoniacal ammonium molybdate solution followed by removal of the magnesium from the solution.
Sources of molybdenum such as impure or technical grade molybdenum trioxide are often contaminated with phosphorus and/or arsenic. These impurities are difficult to remove and are often present as contaminants in molybdenum compounds where they result in inferior quality of the molybdenum products. For example, phosphorus in ammonium dimolybdate shows up in the subsequently produced molybdenum metal. Phosphorus at a level of about 40 ppm in molybdenum powder causes a decrease in the rolling efficiency of molybdenum.
Therefore, a method to remove impurities such as phosphorus and arsenic from molybdenum would be advantageous because it would allow processing of a wide variety of molybdenum sources.
U.S. Pat. No. 3,829,550 discloses a process for producing a high purity molybdenum trioxide and/or ammonium molybdate product whereby an oxidized molybdenite concentrate is subjected to an ammonium hydroxide leaching step including a digestion phase in which an oxidation of some of the impurities therein, particularly iron, is effected, resulting in a coprecipitation of iron and aluminum hydroxide, together with other impurities including lead, bismuth, tin, arsenic, phosphorus, soluble silica, and the like. The resultant aqueous solution containing ammonium molybdate is filtered and thereafter crystallized, followed by calcining to produce a high purity molybdenum trioxide. This molybdenum trioxide can be further purified by digestion in a dilute nitric acid solution to effect a further leaching of residual contaminants, and thereafter the molybdenum trioxide is redissolved in an aqueous ammonium hydroxide solution which is filtered and subsequently crystallized to produce a high purity ammonium molybdate.